Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass into bio-oil. Typical hydrothermal conditions include temperatures from 523-647 K and operating pressures from 4-22 MPa (Elliott et al., Bioresource Technology 2015, 178:147-156). The resulting bio-oil, however, is often viscous and includes undesirably high levels of oxygen. The crude bio-oil components include, for example, acids, alcohols, cyclic ketones, phenols, methoxy-phenols, naphthols, and/or benzofurans. The crude bio-oil is hydrotreated to reduce the viscosity and oxygen content for subsequent fuel use.